Ahhh, you know? I have thought about the reason as to why they are going about this method of limited purchases for clothes and while I know that this may sound like perhaps that I'm making them out to be "the bad guy" here whichever way you may view this, but my initial opinionated conclusion to this came to that of, with us players not being able to buy everything at once because of how large the clothing lineup is now in availability, it then in turn allows the devs to not focus on the notion of needing to make more clothes for us players.
In other words, by not buying everything at once so that players don't get tired and ask for new clothing for every update they give due to the devs possibly not wanting to actually do that and it maybe being a hassle since offering new designs would need to be saved for another AC game in the future, it allows them to work on other projects for the game updates (though so far, every update was a rehash of what we had in NL and we didn't really have any new content...) or simply not want to be bothered by fans whining about how quickly the Able Sisters became "obselete"
In my opinion, it is possibly either that the
devs are truly not intentionally wanting to have to make new clothes for a future update and are trying to stretch out something new for players to buy every time they log on or they simply just do not want to hear the complaints of players saying "We need more new clothes!" or "I don't even go into the Able Sisters anymore what's the point since I bought everything?" kind of shtick.
Personally? Most players nowadays create their own clothing mostly now anyways as finding creators from the kiosk in the Able Sister's is a much more easier and a very grand idea, however, while I do think that the majority of fans prefer creating their own clothing as opposed to buying it unless for catalog purposes, in the end, players still can't create pants, shoes/boots/sandals or other accessories of that nature that the Able Sister's is able to provide to us with, making the Able Sister's very much relevant still no matter how you look at it.
Ahh, I guess long story short? It has to do with a bunch of variables that all tie into the many factors consisting of what the devs are trying to achieve, but overall, I feel the intention of not being able to buy as much as we want is none the more that of...a sly intention? At least, this I what I make of it.